Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

Do not click the Reset button on the Safe mode start window or otherwise make changes.

You can check for problems caused by recent Flash updates and try these:
*disable protected mode in Flash 11.3 and later
*disable hardware acceleration in the Flash plugin
*https://support.mozilla.org/kb/flash-113-doesnt-load-video-firefox
*http://kb.mozillazine.org/Flash
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Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
*Do not click the Reset button on the Safe mode start window or otherwise make changes.
*https://support.mozilla.org/kb/Safe+Mode
*https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes